The molding of lignocellulosic and lignocellulosic containing fibers, particles or layers to form composite articles is known. The binders which normally are used are the synthetic resin glues such as a solution of urea-formaldehyde or phenol-formaldehyde resin in water. Composite products containing lignocellulose produced in this way lack durability and are susceptible to moisture conditions and deterioration in certain building purposes to which they may be subjected.
Di- and polyisocyanates as binders for lignocellulose materials have been proposed and are known to give products of increased stability and mechanical strength. Process technical advantages of polyisocyanates have been disclosed in German Offlegungsschrift No. 2,109,686. Two problems associated with the use of polyisocyanate binders are (1) even at reduced binder use levels, the cost of polyisocyanates as compared to the urea-formaldehyde or phenol-formaldehyde resin binders is unfavorable and (2) the viscosity of the normally employed polyisocyanate binders are much higher than the synthetic glue resins and are therefore difficult to apply to the lignocellulose with current application equipment or methods. The advent of technology to emulsify the polyisocyanates in water provided a binder product with reduced viscosity but with a service life of only 2 to 4 hours.
In a co-pending application of James M. Gaul and Tinh Nguyen, Ser. No. 293,564, filed Aug. 17, 1981 an organic polyisocyanate-liquid alkylene carbonate adhesive binder composition is described. In addition to other improved properties, the added liquid cyclic alkylene carbonate, such as propylene carbonate, imparted a reduced viscosity to the binder system allowing the use of present application technology. Because of its crystalline properties and melting point of 36.4.degree. C., ethylene carbonate per se, cannot be sprayed or employed with the polyisocyanates especially the solid or semi-solid di- or polyisocyanates without the application of heat and other costly techniques. It has been discovered however that ethylene carbonate can be completely dissolved in propylene carbonate at an ethylene carbonate weight ratio of from 5 to 80 weight percent at ambient (25.degree.-30.degree. C.) or room temperature to form a mixture which remains liquid and which may be used with an organic di- or polyisocyanate to give an improved adhesive binder composition for the preparation of compression moldable lignocellulosic materials.
The present invention comprises an improved polyisocyanate based adhesive binder composition that has been extended through the addition of a liquid ethylene-carbonate-propylene carbonate mixture and optionally lignin and at the same time avoids the above mentioned disadvantages.